1. Field of the Invention
The present invention relates generally to memory devices, and more particularly, to a method of fabricating the fixed layer of a magnetic random-access memory (MRAM) device.
2. Description of the Related Art
Magnetic random-access memory (MRAM) cells provide non-volatile memory and typically include a fixed layer and a free layer in a stack configuration. The fixed layer can comprise several sublayers and is magnetized to have a fixed magnetization vector under circuit operation conditions. The free layer has a magnetization vector which is switchable in circuit operation between two opposite orientations which are parallel and anti-parallel to the magnetization vector of the fixed layer. Writing information to the MRAM cell corresponds to magnetizing the free layer into either the parallel or anti-parallel configuration. The resistance of the MRAM cell in the parallel state is different from that in the anti-parallel state, and this resistive difference can be detected to read information from the MRAM cell.
A general configuration for an MRAM cell 2 over a substrate 10 is schematically illustrated in FIG. 1. The fixed layer 20 comprises an antiferromagnetic pinning layer 22 over the substrate 10, a ferromagnetic pinned layer 24 having a first thickness t1 on the pinning layer 22, an antiferromagnetic spacer layer 26 over the pinned layer 24, and a ferromagnetic reference layer 28 having a second thickness t2 over the spacer layer 26. This configuration of the fixed layer 20 is an example of a structure typically called a synthetic antiferromagnetic (SAF) layer. The free layer 30 comprises a ferromagnetic material and is spaced from the fixed layer 20 by a non-magnetic tunneling layer 40 (e.g., AlOx) in the example of a tunneling magnetoresistive (TMR) design.
The orientations of the magnetization vectors of the pinned layer 24 and the reference layer 28 of the fixed layer 20 are set during the fabrication process. Often, this orientation is accomplished by an annealing sequence in which the MRAM cell 2 is cooled through a blocking temperature of the antiferromagnetic material of the pinning layer 22 while a magnetic field is applied to the MRAM cell 2. Above the blocking temperature, the spins of the atoms in the pinning layer 22 are free to orient under the influence of the magnetic field so as to minimize the energy of the pinning layer 22. Below the blocking temperature, the spin configuration of the pinning layer 22 is locked in. Since the pinned layer 24 is exchange coupled to the pinning layer 22, and the reference layer 28 is exchange coupled to the pinned layer 24 via the spacer layer 26, this procedure of field cooling (cooling the fixed layer 20 from above the blocking temperature to room temperature while in an applied field) sets the orientation of the magnetization of the fixed layer 20.